Replace guarded list length field loads with constants

runtime/vm/intermediate_language.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/intermediate_language.h"
 
 #include "vm/bigint_operations.h"
 #include "vm/bit_vector.h"
 #include "vm/dart_entry.h"
 #include "vm/flow_graph_allocator.h"
@@ skipping 1348 matching lines @@
 
   // For fixed length arrays if the array is the result of a known constructor
   // call we can replace the length load with the length argument passed to
   // the constructor.
   StaticCallInstr* call = instance()->definition()->AsStaticCall();
   if ((call != NULL) &&
       call->is_known_list_constructor() &&
       IsFixedLengthArrayCid(call->Type()->ToCid())) {
     return call->ArgumentAt(1);
   }
+  // For arrays with guarded lengths, replace the length load
+  // with a constant.
+  LoadFieldInstr* load_array = instance()->definition()->AsLoadField();
+  if (load_array != NULL) {
+    const Field* field = load_array->field();

[srdjan, 2013/08/28 15:15:08]

'field' can be null, do you need to check?

[Cutch, 2013/08/29 07:23:26]

Done. Conceptual/context question: What does a LoadFieldInstr with a NULL field
indicate?

+    if (field->guarded_list_length() >= 0) {
+      return flow_graph->GetConstant(
+          Smi::Handle(Smi::New(field->guarded_list_length())));
+    }
+  }
   return this;
 }
 
 
 Definition* AssertBooleanInstr::Canonicalize(FlowGraph* flow_graph) {
   if (FLAG_eliminate_type_checks && (value()->Type()->ToCid() == kBoolCid)) {
     return value()->definition();
   }
 
   return this;
@@ skipping 1272 matching lines @@
     default:
       UNREACHABLE();
   }
   return kPowRuntimeEntry;
 }
 
 
 #undef __
 
 }  // namespace dart